I feel like this is standard since 2015
I remember that around 2011-2012 IIRC, back when WhatsApp used to cost 99 cents, half my friends actually paid for it.
The next few years were spent trying to get people to migrate out of other apps (like Facebook Messenger) into it. By 2015 there were very very few people using mainly FB in my circle.
I miss being part of something that felt like a community. I don't know where to find that anymore.
In my (limited) experience with Discord, it has worked exactly like a group chat: each "server" has its own fixed membership, and there does not appear to be any larger community. People don't come and go; I have never met anyone on a discord channel who wasn't already part of the group.
Telegram I have never tried; I thought it was a WhatsApp competitor, and I don't know anyone who uses it.
For discord, it's always an external community for my servers. Either software (home assistant, and a related software netdaemon), a metal review blog (angry metal guy), a game company (owlcat) or a twitch stream (a food truck and a cooking stream). Only my TTRPG group is a closed group.